The seven-day average of migrant apprehensions has dropped more than 40% to less than 2,400 encounters per day since President Joe Biden’s executive action barring asylum at the US southern border went into effect about three weeks ago, the Department of Homeland Security said.

Border authorities apprehended about 1,600 migrants along the US southern border on Monday, a drop from about 1,900 apprehensions on Sunday, a source familiar with the data said.

Despite the significant drop, Biden’s executive order remains in effect, DHS said. Senior administration officials have said the measure would be lifted when there’s a daily average of less than 1,500 encounters between ports of entry.

The latest figures, coming on the heels of Customs and Border Protection statistics that also found a sharp decrease in encounters since the asylum restrictions went into effect, are a promising sign that Biden’s actions to address the ongoing crisis at the border are working, the administration said.

“We indeed have seen a tremendous success early on, and I should emphasize that it is early on in our implementation of the president’s proclamation and our accompanying regulation,” Homeland Security Secretary Alejandro Mayorkas told CNN’s Kate Bolduan on “CNN News Central” Wednesday, adding that the executive order is “no substitute for congressional action.”

Under the rules of the sweeping executive order, migrants who cross the US-Mexico border illegally are barred from seeking asylum once a daily threshold is met. And unless they meet certain exceptions, they could be returned to Mexico or to their country of origin.

DHS has removed or returned more than 24,000 people in more than 100 repatriation flights to more than 20 countries, according to the agency. The swift removal of migrants has led to a more than 65% decrease in the number of migrants released into the country pending immigration proceedings, DHS said.

More Border Patrol agents have returned to the field and to the front lines of border security operations, including the interdiction of individuals who pose a threat to public safety, according to DHS.

Enhanced screening measures have helped identify known or suspected gang members, including members of the Venezuelan “Tren de Aragua” gang, DHS said. Anyone confirmed to be a gang member is detained and either referred for criminal prosecution or placed on expedited removal proceedings, according to the agency.

Responding to the new figures from DHS, Lee Gelernt, deputy director of the American Civil Liberties Union’s Immigrants’ Rights Project, warned that, under the new restrictions, “migrants with credible claims of danger are being denied an asylum screening.”

“In the end, the ban is unlikely to have much impact on the numbers, but if it does deter people in danger from seeking safety here, that’s deeply troubling,” said Gelernt, who is the lead attorney in an ACLU lawsuit over the executive order.

Biden’s executive action sought to address an ongoing crisis that has become a significant political challenge. It was the administration’s most dramatic move on the US southern border and used the same authority former President Donald Trump tried to use in office.

The issue has grown in urgency in recent weeks ahead of the CNN Presidential Debate between Biden and Trump — at which immigration is likely to be a major issue — as well as in light of the murder of a 12-year-old Houston girl, allegedly by two men who were in the US illegally. Trump said over the weekend that if he were president, the two men “would not have been in our country.”

“The criminals who committed these heinous acts must be held responsible with the fullest force of the law,” Mayorkas, who will travel to Tucson, Arizona, later Wednesday to review border enforcement efforts, said on CNN. “We will hold the criminals accountable.”

CNN’s Samantha Waldenberg and Sam Simpson contributed to this report.